Several processes have been used for physically coating metal surfaces, including aluminum and aluminum alloy surfaces. One method known as cathodic sputtering involves "atomizing" or "bombarding" a suitable target with high energy gas ions, which are accelerated in an electric field. Using cathodic sputtering, the surface atoms are displaced from the target and deposited on the surface to be coated where they form a layer.
In a modification of the cathodic sputtering method, a magnetic field is applied to the target to increase the sputtering rate significantly, according to the Penning principle. This modified cathodic sputtering technique is referred to as magnetron sputtering. In magnetron sputtering, a small portion of the energy which is applied to the target is converted into kinetic energy with the majority of energy transformed into heat. To effectively displace atoms from the surface of the target, the excess heat must be dissipated by intensively cooling the target.
One serious consequence of bombardment heating, arising from, for example, magnetron sputtering, is a marked deterioration in the mechanical stability, including the shape, of the aluminum and aluminum alloy metal target.
To overcome this deterioration in metal stability, and to allow for sputtering with high energy output, prior art teaches composite targets with backing plates, which impart high mechanical stability and good heat conductivity, to improve the physical characteristics of the composite target and its performance during bombardment heating and minimize deformation.
Backing plates taught by the prior art for aluminum and aluminum alloy targets typically consist of copper. The two metals, i.e. aluminum and copper, or aluminum alloy and copper, are normally joined by conventional methods such as gluing, soldering or explosive plating. However, such targets cannot be roll-bonded because, as a practical matter, the thickness of the copper layer in the backing plate cannot exceed 3 mm. Such thicknesses, i.e. thinner than 3 mm, limit the degree of mechanical stability and resistance to heat deformation which can be imparted to the aluminum or aluminum alloy layer by the copper backing. Thus, such prior art targets which employ copper backings suffer serious disadvantages.